The present invention relates to a position, velocity or force control apparatus for use in a robot manipulator, machine tool and XY table etc.
Predictable or unpredictable disturbance torque is generated at a motor axis in a mechanical drive system such as a robot or machine tool. The disturbance torque causes disadvantage degradation of controllability such as to hinder normal mechanical drive, induce vibration, and induce tracking error from a target projectory.
Conventionally, as shown in FIG. 2, position and velocity control are performed by a feedback control loop utilizing feedback signals in the form of a position detection signal 6 from a position detector and a velocity signal 14 of a control object 2. In this configuration, the feedback loop operates to automatically eliminate the disturbance torque 7, and the feedback loop is constructed such that the loop gain is set as great as possible to eliminate deviation.
FIG. 5 is a diagram showing a transfer function block corresponding to the FIG. 2 conventional structure. As shown in this block diagram, an integral compensation, block 11 is added in the feedback loop to eliminate standing deviation or difference between a position command signal 5 and a detected position represented by the position detection signal 6. However, the stable operating zone of the control system is limited depending on the characteristics of a control object 2. Therefore, simply raising the loop gains K.sub.1, K.sub.2 and K.sub.3 cannot ensure sufficient performance.
There have been proposed various types of the conventional control systems constructed which take into account dynamic characteristic change for predictable disturbance torque. However, these control systems cannot maintain the same control characteristic for adequately controlling unpredictable disturbance torque.
In view of this, as shown in FIG. 3, there has been proposed a conventional velocity control system which utilizes a disturbance torque observer 4 operative to estimate a disturbance torque 7 based on a drive current or a drive torque signal 8 generated in an actuator and a movement velocity signal 14 so that the disturbance torque 7 is treated as part of a control object 2. As shown is FIGS. 2 and 3, a velocity feedback loop is incorporated in a position control processor 3 to construct a position control system so as to improve the controllability.
Because of the desired reduction in size and weight, compact type robots and machine tools are generally not provided with a velocity detector such as a tachometer, but only have a position detector. In such a case, a velocity signal 14 is not directly obtained from the control object, but a differential value of successive position detection data is used as a velocity detection data as shown by the dashed line 14a appearing in FIG. 3. The differential operation by an analog differentiation circuit may not be practical because the analog circuit is susceptive of high frequency noise. Alternatively, the control object is monitored by a position detector which generates pulses indicative of the object position. In such a case, a velocity detection circuit is generally used to produce a voltage proportional to a frequency of the pulses. However, such a velocity detection circuit produces a ripple in a relatively low velocity range, thereby causing degradation of detection accuracy and causing vibration. A low-pass filter may be inserted to remove a ripple of the velocity detection circuit. However the low-pass filter causes a response delay to thereby degrade the controllability. Further, in a digital servo control system, velocity data is approximated by a step difference between the current position data at one sampling time and the preceding position data at an immediately previous sampling time. However, a ripple may also develops in a relatively low velocity range to cause vibration and noise.
As noted above, in the conventional position control system having only a position detector, there are practical problems, in that the observer operates to estimate a disturbance torque based on a drive torque signal and a velocity signal in the form of a differential value of the position detection data.